Saishuu Shiken Kujira Progressive

In the past Mutsumi (Mucchan) and Sae (Sacchan) met. Enjoying the time together, they decided to meet again and chose a place. Mucchan, however, never came to that place.
A few years later he passed through that place on his way to school, and met Sacchan, who had been waiting for him for all those years. And so, they start playing together again. However, it won't last so long, because...